With the development of telecommunication and the growth of network capacity, reliability becomes more and more important. It has been a consideration for each apparatus manufacturer how to prevent single point failure from causing service interruption in large area.
Iu-Flex technology is a disaster recovery mechanism in circuit switched domain introduced by 3GPP in Release 5, wherein the concept of “pool” is defined, and a pool includes a plurality of RNCs (Radio Network Controllers)/BSCs (Base Station Controllers) and a plurality of CN (Core Network) Elements. Each RNC/BSC node may be connected to a plurality of CN Network Elements.
Normally, Iu-Flex mechanism may be ensured by NRI (Network Resource Indicator) mode, when a certain subscriber roams in an area administrated by a pool, all of the CN processing are performed on a specific CN node in the pool; that is, for a single subscriber, the subscriber has only one CN Processing Node, thus frequent location update can be avoided. However, for all the subscribers in the pool, they are distributed to different CN Nodes in the pool, thus load sharing is achieved; this distribution of subscribers among CN nodes in the pool should be determined by the RNC when the subscribers initially register according to some IMSI-HASH algorithm, and all the subsequent NAS (Non Access Stratum) processes going to the same CN noted are ensured by NRI. In abnormal circumstances, if failure occurs on a certain CN node in the pool, the RNC uses some mechanism to change the CN node which is supposed to handle messages from a subscriber, e.g. in normal case, the register message initiated from subscriber A is determined to go to MSC1 (Mobile Switching Center 1) by RNC, then MSC1 breaks down and RNC chooses MSC2 for subscriber A. Afterwards when subscriber A sends out a register message, RNC will send it to MSC2. The term “re-distribute load” is used to describe the change of CN node hereinafter.
Similarly, a mechanism named A-Flex, may also be disposed in a CDMA2000 network; that is, a BSC in a CDMA2000 network may be connected to a plurality of MSCs/VLRs (Mobile Switching Centers/Visit Location Registers). Similar to the concept of “pool” in 3GPP, the A-Flex area in the present invention is a set of elements which includes at least one BSC (usually a plurality of BSCs) and at least two MSCs/VLRs serving the BSCs simultaneously. Each BSC is connected with all the MSCs/VLRs in the area. FIG. 1 shows a flow chart of location registration in the prior art, which is illustrated as following. A BSC sends a location registration message, Location Update Request, to MSC1/VLR1 for location registration; the MSC1/VLR1 then detects that change occurs in the MSC/VLR in which the subscriber exists, and sends a registration notification request (REGNOT) carrying location information of the subscriber to the HLR (Home Location Register); the HLR returns a registration notification response (regnot) to MSC1/VLR1; and MSC1/VLR1 returns a message Location Update Accept, which indicates that the location information has been updated, to the BSC.
However, in the prior art, no matter whether it is Iu-Flex or A-Flex, it cannot avoid communication interruption when the subscriber's serving MSC/VLR suddenly breaks down and is not able to process signaling. It is imagined that various reasons may cause an MSC/VLR breaking down suddenly, such as too heavy traffic, burst failure and so on. In Iu-Flex mechanism, after it is determined to re-distribute load, because the RNC only re-determines the MSC/VLR to which a message related to a subscriber should be sent when the location is updated, the load re-distribution could not be really achieved until the next location update, which means the originating call and terminating call can not be processed by the new MSC/VLR until the location of the subscriber is updated (messages are sent to the original MSC/VLR according to NRI, original old MSC/VLR has broken down and could not process signaling). As a result, if the original MSC/VLR breaks down suddenly, signaling may not be processed normally, and at this point, subscribers registered on the MSC/VLR will be in a communication failure state before next location updating; the communication failure time period may depend on when the next periodicity location update comes or when other operations (e.g. network forced location registration, automatic location registration triggered by the subscriber moving to a new location area) that may trigger location registration occur.